I am trying to use Salesforce DX so I can create scratch orgs for my managed package, but I need to create a scratch org w/ the same namespace. I found this article https://developer.salesforce.com/docs/atlas.en-us.packagingGuide.meta/packagingGuide/sfdx_dev_reg_namespace.htm but I do not have an app called "Namespace Registries" (step 3). 

How do I get that app or how do I create a scratch org with the same namespace prefix as my managed package?
Thanks 

I installed Developer Hub Trial Org 64-bit Windows option.
I created a trial project with: 
     sfdx force:project:create -n geolocation

But, when I try to create the scratch org with:
      sfdx force:org:create -s -f config/project-scratch-def.json -a GeoAppScratch
I get this:
     ERROR running force:org:create: You do  not have access to the [scratchorgiinfo] object

I've tried several suggested fixes that I found when I searched for the above error. None has resolved the issue for me.